The present disclosure relates to a method for monitoring the rotating spray body of a rotary atomizer, e.g., the fixing of the spray body, as well as a corresponding rotary atomizer according to the preamble of the independent patent claims. Rotary atomizers of this type may be used for conventionally electrostatic series coating of workpieces such as vehicle bodies, and are conventionally mounted on painting robots or similar automatic coating machines.
Some known bell cups, which serve as spray bodies, of the electrostatic rotary atomizers (for example, as described in DE 43 06 799 and corresponding U.S. Pat. No. 5,727,735, EP 0 715 896 and corresponding U.S. Pat. No. 5,707,009, DE 10 2005 015 604 and corresponding U.S. Pat. Pub. Nos. 2006/0219816 and 2010/0143599, and DE 10 2006 057 596 and corresponding U.S. Pat. Pub. No. 2011/0000974) are employed in an automatic series coating of workpieces and may have a cylindrical hub member provided with an external thread, which hub member is screwed into the open end face of the hollow shaft of the drive motor consisting of an air turbine and can for example be unscrewed for maintenance purposes or for replacement with a new bell cup. Due to the high rotational speeds of the air turbine, e.g., in the range of more than 50,000 rpm, a precise centering and balancing of the bell cup in relation to the hollow shaft axis is important for this fixing apparatus. Accordingly, the hub member of the bell cup may have a conical part which bears against a correspondingly conical region of the internal wall of the hollow shaft for forming a centering cone. In the case of other known rotary atomizers (EP 1 266 695 and corresponding U.S. Pat. Nos. 6,811,094 and 6,988,673), by contrast, the hub member of the bell cup may have an internal thread, using which it is screwed onto an external thread at the end of the hollow shaft.
In addition to the centering and balancing, the apparatuses for fixing a bell cup on the drive shaft thereof must satisfy further conditions, such as a secure seat for reliable transmission of torques in both rotational directions when accelerating or braking, low space requirement, low risk of contamination, e.g. by paint mist and simple cleaning, as well as not least the option of fast and simple mounting and demounting.
In the case of the known rotary atomizers, the problem exists that the releasable fixing apparatus can unintentionally come loose itself in the event of faults. Such faults may have different causes, such as e.g. wear of the turbine, damage due to collision of the bell cup with the workpiece to be coated or due to improper handling, unbalancing of the bell cup due to damage, due to incorrect screwing on or due to contamination etc. and in each case can lead to a sudden sharp braking or locking of the shaft. The risk of an unintentional loosening of the bell cup can, in the case of a bell cup which is screwed on or in, also arise in the case of strong acceleration or deceleration of the bell cup, depending on the thread direction (right or left). In any case, the bell cup rotating at high rotational speed and unscrewing itself due to its kinetic energy can be spun off from the atomizer with the consequence of considerable mechanical damage and accident risks.
Various constructional options for preventing the spinning off of the bell cup are described in EP 1 674 161. Above all in the case of complete automation of the coating operation, they are not sufficient however, as although the mechanical damage and accident risks mentioned are prevented, coating errors due to the bell cup disengaging from the correct position during the production process cannot be excluded. Typical malfunctions of atomizers are, for example, an unstable spray jet or interruption of the atomization with the consequence of workpieces which are defective, as they have not been coated flawlessly.
Methods known from the generic prior art for the monitoring of the presence or fixing of components use electrical sensors and/or optical components. Due to the high voltage at electrostatic atomizers with the necessity of insulation measures connected therewith, electrical connections and electrical components in atomizers are not used for the purpose considered here, however, and due to the generation of paint aerosols by means of the atomizer, optical components for fixing monitoring are also unsuitable on account of the contamination sensitivity thereof.
A pneumatic system for monitoring the presence or the determination of an incorrect fixing of the bell cup of a rotary atomizer, which is fixed on the drive shaft by means of axial magnetic force, is known from EP 1 789 200 and corresponding U.S. Pat. No. 7,770,826. The drive shaft has an axial compression bearing in which an air cushion counteracting the magnetic force is generated by an own compressed air source thereof, the pressure of which air cushion is monitored using a sensor. Various problems arise in practice with this system, such as, e.g., the necessity of the special compressed air source, which additionally must be regulated precisely to prevent measurement errors and in the event of overpressure can loosen the magnet fixing. Also difficult is a precise and error-free pneumatic signal transmission from the axial compression bearing to the sensor, wherein functional faults due to blocking of the sensor channel or bending of the air hose are possible. Further, the compressed air flowing out of the bearing can impair atomizer functions such as the brush size for example. Incidentally, no rotation detection takes place in the known systems either.
Accordingly, there is a need for an improved bell cup and/or method of using the same which inhibits or prevents malfunctions of rotary atomizers, including corresponding coating errors, caused by the bell cup, particularly in the case of the loosening of the fixing thereof, without the disadvantages of the known systems.